1. Field of the Invention
The present invention relates to a laminated piezoelectric element particularly useful as an actuator.
2. Description of the Related Art
Currently, in answer to the demand for high speed printing, a laminated piezoelectric element capable of very quick response when the voltage applied thereto is changed, is used as an actuator in a printing head. Nevertheless, in laminated piezoelectric elements, insulation breakdowns and current leaks occur due to recesses or pores present on the surface of piezoelectric ceramics, and accordingly, it is difficult to obtain a desired reliability. Therefore, there is a need for a highly reliable laminated piezoelectric element which is resistant to insulation breakdowns.
Conventional laminated piezoelectric elements are manufactured by preparing a plurality of green sheets made of piezoelectric ceramics, coating a metal paste film on one of the surfaces of each green sheet, to thus form an inner electrode, laminating and sintering the plurality of the green sheets, cutting the laminate to a predetermined size, and connecting positive and negative outer electrodes to the inner electrodes, which are exposed at both sides of the cut laminate, so that a voltage can be applied to each of the piezoelectric ceramics sandwiched between the inner electrodes. The thus formed laminated piezoelectric element is often further encapsulated by a resin for insulation purposes.
Nevertheless, piezoelectric ceramics contain many pores in the body thereof, which are formed during the fabrication thereof, and recesses are formed on the surface of the piezoelectric ceramics when such piezoelectric ceramics are cut, and water in the air is captured and adsorbed in such recesses if the laminate is used in a high temperature and high humidity atmosphere. Even if a resin coating is provided around the laminated piezoelectric element, the resin coating may tend to peel from the surface of the piezoelectric ceramics during repeated operation of the piezoelectric element, and thus water can penetrate past the resin coating and reach the surface of the piezoelectric ceramics and, in this case also water is captured and adsorbed in recesses on the surface of the piezoelectric ceramics. If water is present in the recesses on the surface of the piezoelectric ceramics, a current path can be formed through the water layer, whereby the insulation between the inner electrodes is broken and a current occurs, and accordingly, a voltage applied to the piezoelectric ceramic becomes inappropriate for the desired purpose, and thus the reliability of the laminated piezoelectric element is lessened.